Difficulty has to increase 7.5x current values to make FPGA's more profitable at current prices.
At that point the economies make no sense. At 3.3M difficulty, with your 250M FPGA you will be producing 0.08 BTC per day, so at $10/BTC you would pay off your FPGA in 625 days, in order to pay off your FPGA in the same 30 days as you can pay off your GPU currently BTC would have to be worth $208/BTC. Because entrance to FPGAs is MUCH higher than GPUs that will push their usage back. And you forgot to account for people with free power which may very well eliminate FPGAs usage all together. FPGAs will probably never be popular for mining due to slowing difficulty and ever lower ROI.
625 day payoff is not very long. 50%+ ROI in one year is amazing. I just bought a house and it's only producing a 25% ROI per year.
My point is a company could make this investment tomorrow, have the system online in a month, and be way out in front in the development of FPGA mining while making all our GPU's worthless. I know we can sell, yadda yadda, but 10,000 GPU's being dumped on ebay at the same time will not sell for what you think.
Yes that is very true it is an amazing ROI, but the market is so volatile and young, putting that much money into it is a huge gamble, you don't know if BTC is going to be worth $1 or $1000 in 2 years, or even if it lasts that long. I think if you explained what bitcoin is and the small history no one in their right mind would invest, there are just too many unknowns.